This application claims the priority of Japanese Patent applications No. 10-64841 filed on Feb. 27, 1998 and No. 10-66194 filed on Mar. 2, 1998 which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light control system for electronic endoscopes, and more particularly, to an arrangement for regulating the amount of light emitted to the object under observation by the use of a diaphragm to obtain sharp images even during observation of distant objects.
2. Description of the Prior Art
In the electronic endoscope systems, a charge coupled device (CCD), for example, is used as a solid-state image sensor. CCD relies on photoelectric conversion elements for reading stored electric charges corresponding individual pixels to obtain picture signals (video signals). Besides, a light source section (or a separate light source unit) is provided to supply illumination light inside the body under observation. In this light source section it is designed to provide the optimum amount of light by means of a diaphragm control mechanism.
That is, the video signal outputted from the CCD is supplied, for example, to a digital video processor (DVP), etc. where a color-difference signal, luminance signal, etc. are generated. The luminance signal is supplied to the diaphragm drive circuit as a control signal for adjusting the brightness of the screen. The diaphragm drive circuit drives the diaphragm so as to keep the luminance signal constant; and acts to open the diaphragm if the luminance signal is below a given reference value, and acts to close the diaphragm if the luminance signal is above the reference value. This variable control of the diaphragm opening varies the amount of light emitted from the distal end of the endoscope, allowing the brightness of images to be kept constant.
Also, an electronic shutter function may be used for drive control of the CCD described above. It regulates the amount of received light (light exposure) via variable control of pixel data, i.e., the storage time of the electric charges. More particularly, the video signal outputted from the CCD is converted into a color-difference signal and luminance signal by means of, for example, a digital video processor (DVP), and the electronic shutter is operated according to this luminance signal. For example, if the luminance signal is below a given reference value, the shutter speed is decreased to increase the amount of light received, and if the luminance signal is above a given reference value, the shutter speed is increased to decrease the amount of light received. This provides the exposure suitable for the object under observation, maintaining the brightness of the screen in good condition.
However, conventional electronic endoscope systems have a problem that if the object under observation is located at a significant distance in a deep cavity or the like, the light emitted from the distal end of the endoscope will not reach the object under observation, resulting in a deficiency in the amount of light (received by the CCD), and thus inability to provide an image bright enough for observation.
Possible means of solving this problem includes, the use of a high-output, bright lamp, and enhancement of the lamp""s output or capacity through application of a higher voltage. However, the use of a high-output lamp will entail high cost and abuse of a lamp in excess of its capacity will shorten its life.
Besides, when using diaphragm control with increased light output, the diaphragm must reduce the amount of emitted light greatly during close range observation, making light control unstable. This is because the diaphragm allows the amount of emitted light to be regulated roughly during observation at narrowed aperture, compared to observation at wide aperture.
The present invention has been made in view of the above problems. The object of the present invention is to offer a light control system for electronic endoscopes that can provide sufficient brightness for long range observation and provide stable amount of light for close range observation, and reduce costs, and prolong lamp life.
To attain the above object, the present invention comprises a light source for illuminating the object under observation, a light-source voltage control circuit for controlling the light-source voltage to change the brightness of the said light source itself, means for detecting the amount of light received which controls said light-source voltage control circuit, supplies a constant light-source voltage to said light source under normal conditions, and detects the amount of light received by said image sensor, and a control means which raises said light-source voltage only when it is judged, based on the results obtained by said means of detection, that the amount of light received by said image sensor will be insufficient.
Another invention comprises a light source for illuminating the object under observation, a diaphragm for regulating the light output of the light source, a diaphragm drive circuit for driving the diaphragm variably, a light-source voltage control circuit for controlling the voltage of the light source to change the brightness of the light source, and a control means which controls this light-source voltage control circuit by supplying a constant voltage to the light source under normal conditions and raising the voltage of the light source only when it is judged, based on the detected amount of light received by the image sensor, that the amount of light received by the image sensor will be insufficient even with the diaphragm fully open.
In this configuration, the control means detects the amount of light received by the image sensor based on the luminance signal of an image and outputs the control signal for luminance regulation to the diaphragm drive circuit, which drives the diaphragm variably so as to keep the brightness of the image constant. And, if it is judged that the brightness is not sufficient even when the diaphragm is fully open, such a command signal is outputted to the light-source voltage control circuit, instructing it to raise the voltage of the light source. Since this raises the output of the light source higher than usual, sufficient brightness can be obtained even for images of distant objects observed.
Another invention comprises a light source for illuminating the object under observation, an electronic shutter circuit for regulating, by means of electronic shutter speed, the amount of light received by the image sensor, and a control means which controls this light-source voltage control circuit by supplying a constant voltage to the light source under normal conditions and raising the voltage of the light source only when it is judged, based on the detected amount of light received by the image sensor, that the amount of light received by the image sensor will be insufficient even with the electronic shutter set at the lowest speed.
In this configuration, the control means detects the amount of light received by the image sensor based on the luminance signal of an image and outputs the control signal for luminance regulation to the electronic shutter circuit (CCD drive circuit), which variably sets the shutter speed to keep the brightness of the image constant. If it is judged that the brightness is not sufficient even at the lowest electronic shutter speed (with the electronic shutter fully open), it issues a command signal to the light-source voltage control circuit, instructing it to raise the voltage of the light source. This allows sufficient brightness to be obtained even for images of distant objects observed.